This invention relates generally to optical filters that may be useful for multiplexing and demultiplexing optical signals in wavelength division multiplexed communication networks.
In wavelength division multiplexed optical signals, a plurality of different optical signals, each having a different wavelength, may be multiplexed over the same optical link. At intended destinations, one or more of the wavelength signals may be separated using a demultiplexing technique.
An arrayed waveguide grating, also called a phased arrayed waveguide or phaser, works like a diffraction grating. It may be fabricated as a planar structure including input and output waveguides, input and output slab waveguides, and arrayed waveguides. The length of any arrayed waveguide may differ from adjacent waveguides by a constant ΔL.
The input slab waveguide splits the wavelength channels among the arrayed waveguides. Each portion of the input light traveling through the arrayed waveguide includes all of the wavelengths that have entered the grating. Each wavelength in turn is individually phase shifted. As a result of that phase shift and phase shifts at the input/output slab waveguides, every portion of light at a given wavelength acquires different phase shifts. These portions may interfere at the output slab waveguide, producing a set of maximum light intensities. The direction of each maximum intensity depends on its wavelengths. Thus, each wavelength is directed to an individual output waveguide.
Commercially available arrayed waveguide gratings have Gaussian transmission spectral transfer functions that are easy to manufacture. However, high speed applications usually involve flat or wideband profiles. Currently, such flat spectral shapes may be achieved by introducing a horn taper of various profiles, such as parabolic, exponential, sinc, Y-splitter, and the like, at the free propagation region of the arrayed waveguide grating. However, this approach leads undesirably to higher losses than conventional arrayed waveguide gratings and poses manufacturing challenges since horn tapers are very sensitive to fabrication tolerances.
Thus, there is a need for low loss, wideband or flat top arrayed waveguide gratings.